1. Field of the Invention
This invention relates to a process for the fabrication of flexible tubular structures by extrusion. More specifically this invention relates to an extrusion process for the manufacture of flexible tubular structures with a reinforced or unreinforced polymer base, used for the transport of liquid, gaseous or pulverized fluids, consisting of the application of a film a few microns thick which acts as a sealant barrier to ensure impermeability to the fluids transported. This invention also relates to the flexible tubular structures produced by the process.
Flexible tubular structures for the transport of fluids are most frequently fabricated in long lengths by successive extrusion processes alternating with the establishment of textile or possibly metal reinforcement layers by knitting, wrapping or braiding processes which are customary in both the thermosetting and thermoplastic polymers industry. That is, a flexible tubular assembly is produced by first extruding an inner tube, followed by applying a reinforcement layer, and then applying a covering by extrusion. A significant sector of this industry employs fabrication processes which utilize curing by heat treatment of the assembly of the polymer constituents of the hose, an irreversible transformation which bonds polymers which may be of different types, thanks to their chemical affinity.
In service, the pressures of the fluids carried, as well as the dynamic stresses exerted, govern the selection of the reinforcement elements, while the aggressive chemical or thermal characteristics of the fluid and of the external environment govern the selection of the polymer materials constituting the tube and the coating.
The field in question is more precisely the field of small-diameter tubes, having diameters on the order of one centimeter, used for the transfer either of modern automotive fuels--called oxygenated fuels--with high concentrations of incorporated alcohols, or the transfer of two-phase fluids in cooling systems. The liquid carried behaves like a solvent, and is easily absorbed by the conventional materials used in the internal tubes of the hoses. The specifications--e.g. those issued by the automobile manufacturers--define a very low permeability for such hoses, on the order of a few grams per hour and per square meter.
The internal tubes, made of ordinary or special elastomers, such as nitrile-base elastomers, have permeabilities which are ten times higher than those desired.
2. Background Information
Hose manufacturers have thus made improvements, by this means, in the conventional construction employing extrusion. The principle of this process is described in Canadian Patent No. 951 259 (Goodall Rubber Company) which, while denying the necessity of adherence between constituents, proposes to form a barrier film by immersing the hose in a vaporizable solvent bath. This document proposes an improvement of the impermeability to liquefied petroleum gas (LPG) of a tube made of a polyethylene-butyl mixture by using a polyester film.
Said film is used only to protect the textile reinforcement, which is then external, without attempting to achieve a bond between the constituents.
Likewise, Japanese Patent No. 1 152 061 (Tokai Rubber Industry) proposes to apply the barrier function to the transport of coolant fluids in halogenated rubber tubes by the interposition of a thin layer of polyvinylidene chloride, the purpose of which is to protect the short-fiber reinforcement of the intermediate layer.
This document applies the principle of protection by multiple plastic layers to the conventional fabrication processes employing extrusion.
Japanese Patent No. 1 306 239 (Tokai Rubber Industry) discloses a polyamide resin tube protected by a mixture of this resin with an EVA-type copolymer, without any particular indication concerning the technology to be employed for the use of these multi-layer protections.
On the other hand, Japanese Patent No. 1 259 944 (Kuraray) discloses a fabrication process by spiral winding of thin films around a tube for the transport of propane or air conditioning fluids, whereby the films can be made of ethylene and polyvinyl alcohol copolymers (EVOH), or polyethylene terephthalate (PET). The document relates to the dimensional ratios of the ribbons making up these films, and the rate of their overlap, without going into the bond between the components.
Finally, U.S. Pat. No. 3,927,695 (Union Carbide) applies the improvement of a barrier film to a silicon elastomer tube cured by vulcanizing in hot air and supported, during its fabrication, by a mandrel. The superimposition of the reinforcements is accomplished by spiral windings, and the seal is completed by the spiral wrapping of a polyethylene terephthalate film.
This film is rendered adhesive and suitable for use on high-speed winding machines by surface coating by means of a mixture of an olefin triazine and rubber, the subsequent heat treatment of which makes possible the bond between the silicon tube and the polyethylene terephthalate film, which normally do not adhere to one another.
Several variants are proposed to combine the polyethylene terephthalate barrier film, coated with a mixture of siloxane and triazine, with silicon elastomer tubes, constituted in all cases by superimpositions of spiral windings of calendered layers. Such hoses are realized on a rigid mandrel which rotates, making the high-speed wrapping operation possible. The operation has the disadvantage that it is discontinuous, and that it can only handle a limited length for all the examples cited in said document.
The above documents thus describe different solutions by incorporating, in the tubes, films which are quasi-impermeable to the fluids carried, but none of the proposed solutions would make it possible to achieve the values currently required by specifications--particularly in the automobile industry--either because of the absence of an intimate bond between the components, which leaves room for the diffusion of fluids between said components, or because of a "wick" effect of the reinforcement, which facilitates the penetration of the fluid, once the internal tube exhibits the least porosity, or because of the low effectiveness of the films used as a barrier element, which must have a significant thickness.
In addition, an examination of the prior art shows that although the principle of the installation of a barrier film inside the components of a flexible tube is well established, on the other hand there is no known technique for continuous fabrication which does not require the discontinuous manufacture of a blank, but which still achieves an intimate adherence among the components.
The invention relates not only to the realization of flexible tubes, but also to the realization of any flexible tubular structure such as, to cite several non-restricting examples, expansion compensators or sheathed electric cables, which must be sealed either against the fluids carried or against aggressive external agents such as oxygen or ozone.